1. Field of the Invention
This invention relates to a thermal printer with a thermo-sensitive recording system.
2. Description of the Related Art
Thermo-sensitive recording is suited for highly graded maintenance and has therefore been utilized in many terminal printers including facsimiles. Especially, thermo-transfer type thermo-sensitive recording has recently been developed, making it possible to perform polychrome or full color recording.
Conventionally, a thermal printer is controlled for thermo-sensitive printing as will be described below with reference to FIG. 1.
The thermal printer as diagrammatically shown in FIG. 1 comprises a plurality of heating elements 31 in the form of heating resistors, driver circuits 32 for powering the heating elements 31 to heat them, a latch circuit 33 for applying dot (heating element) data signals to the driver circuits 32, and a shift register 34 for receiving a print data signal containing the dot data signals and applying the dot data signals to the latch circuit 33.
In operation, print data signals for one line are first inputted to the shift register 34. The latch circuit 33 then responds to a strobe signal to latch the print data signal. Subsequently, enable signals are selectively applied to the driver circuits at different phases or timings so that the driver circuits are sequentially actuated to feed currents to the heating elements. As a result, the heating elements are heated in accordance with the dot data signals to perform printing.
During the printing operation, correction data signals in association with the respective heating elements 31 are applied to the shift register 34. The correction data signal is prepared on the basis of a dot data signal for the preceding line (a hysteresis correction data signal) and a neighboring dot correction data signal, and is used in the same manner as in the case of the above printing operation to correct printing.
Problems are encountered in the conventional thermal printer as will be described below with reference to FIG. 2. In high-speed printing, the amount of energy applied for printing is controlled in accordance with contents of the hysteresis correction data signal and neighboring dot correction data signal. As an example, FIG. 2 illustrates a timing chart of one-line printing which is performed in 6.15 msec by using a head of 8 dots/mm density for A4 size paper when the head driving frequency is 1 MHz and 1568 dots (heating elements) of one line are divided into 7 blocks each of which is actuated by an enable signal. The driver circuit is actuated by an enable pulse 1 so as to respond to dot data signals and by an enable pulse 2 to respond to correction data signals, with the result that the two enable pulse can not be applied continuously. This is because dot data signals for one line must be transferred at a time in 1568 .mu.sec. Since, in the conventional thermal printer, paper feeding is affected in timed relationship with each enable signal, the discontinuity of the two pulses 1 and 2 results in a shear in printing.
In addition to the above-mentioned improper application of the hysteresis correction data signal, correct controlling of applied energy can not hitherto been obtained when head temperature and ambient temperature vary. For these reasons, the amount of energy applied for printing can not be controlled properly and accurate printing can not be obtained with the conventional thermal printer.